1. Field of the Invention
The present invention relates to a power filter circuit for protecting electronic equipment from supply system electromagnetic interference, and, more particularly, to sensing circuits capable of determining whether the supply system is properly wired and has a correct or reverse voltage polarity, and switching circuits which connect neutral and ground conductors of the power filter circuit, correct the voltage polarity, or disconnect a load from the supply system in response to the sensing circuit.
2. Description of the Related Art
With the proliferation of highly sensitive electronic and microprocessor-based equipment, it is increasingly important to have filtering devices which protect such equipment from disruptive or damaging supply system current and voltage anomalies at a reasonable cost. Such anomalies, broadly referred to as electromagnetic interference (EMI), include continuous or repetitive interference, such as radio frequency interference (RFI) and power line harmonics, and transient interference, such as short duration overvoltage conditions. To receive power, sensitive electronic equipment is generally connected to an AC supply system via a line conductor which carries an AC line voltage of 120 volts rms (Vrms), a neutral conductor, and a ground conductor. The energy propagation paths through these conductors are conventionally characterized as "normal mode" (i.e., line-to-neutral) and "common mode" (i.e., line-to-ground or neutral-to-ground), and interference occurring over these paths can be characterized as normal mode interference and common mode interference, respectively.
Prior art power filters have employed passive inductive, capacitive and resistive components to address normal mode and common mode RFI and have employed nonlinear devices, such as varistors, gas arresters and silicon avalanche diodes, which conduct only in response to overvoltage conditions to divert overvoltages from sensitive equipment. In general, it has been found that the use of larger capacitor elements between the filter circuit conductors improves low-frequency RFI suppression.
For safety reasons, regulatory bodies such as Underwriters Laboratories, Inc. (UL), have placed a 0.5 mA limit on the steady-state current (leakage current) that a device can conduct to the supply system ground conductor. Consequently, in power filters, common mode circuitry must have a high enough impedance to restrict the flow of leakage current to ground, and this requirement sets a limit on the capacitance of interference-filtering capacitors connected between line and ground and neutral and ground, which in turn limits the filtering of low-frequency common mode interference. While RFI interference on the order of approximately 30 kHz and higher can be filtered by capacitors which meet UL leakage current requirements, equipment can malfunction from interference, such as power line harmonics, at much lower frequencies. In recent years, power line harmonics have come to be recognized as a severe problem due to the proliferation of switch-mode power supplies in all environments and variable-speed drives for motors in industrial environments. Such power line harmonics are generally not significantly attenuated by capacitors which meet UL common mode leakage current requirements.
Since there are different circuit design considerations applicable to suppression of normal mode interference and common mode interference, such as the above-described safety requirements, it is helpful to know whether the AC line voltage is being received with the correct polarity (little or no steady-state voltage across the neutral and ground conductors) or the reverse polarity (little or no steady-state voltage across the line and ground conductors). To this end, modem electrical outlets as well as the plugs of power-sensitive devices are typically configured to mate in only the correct polarity, such that the line and neutral terminals of the 120 Vrms supply system can be connected only to the line and neutral leads of the filtering circuitry, respectively. However, it is not uncommon for the polarity of the power supplied by an AC power network to be the reverse of the indicated polarity, with 120 Vrms being supplied at the neutral terminal of the power outlet instead of the line terminal of the power outlet. Under reverse polarity conditions, the line-to-ground and neutral-to-ground common mode paths are effectively reversed, which, if not properly accounted for in the design of the power filter circuit, creates a risk of excessive leakage current or inadequate EMI protection. Consequently, polarity checking devices that alert the user to a reverse polarity condition in the supply system have been employed in power filter circuits. However, such devices do not automatically correct the reverse polarity condition. Thus, more complicated or expensive power filter circuit designs are typically required to account for the possibility of a reverse polarity condition.
For example, most prior art power filtering circuits generally use an impedance network relying largely on capacitors to suppress common mode interference. Because of the possibility of having 120 Vrms between any two conductors, these devices must use relatively small capacitors between all of the conductors, including the neutral and ground conductors, to meet UL leakage current requirements. Further, these power filter circuits conventionally include clamping devices, such as varistors, connected between each pair of conductors (e.g., in parallel with the capacitors). Below their rated voltage, these clamping devices are non-conductive, thereby presenting an open circuit. Above their rated voltage, these clamping devices conduct, thereby creating a short circuit designed to prevent transients above the rated voltage from reaching the connected electronic equipment. Once again, however, because of the of the possibility of having 120 Vrms between any two conductors, all of the clamping devices, including the clamping device connected between the neutral and ground conductors, must be rated for more than 120 Vrms. This results in an arrangement that can, at best, suppress neutral-to-ground disturbances to about 200 V and, when subjected to the high transient energy levels that often occur on the 120 Vrms supply system, may allow up to 500 V or more to reach connected equipment. For sensitive equipment, this often leads to disruption or damage. Thus, because of the risk of a reverse polarity condition, these devices provide relatively poor common mode suppression of low frequency interference between the neutral and ground conductors and relatively poor common mode suppression of transients between the neutral and ground conductors.
Some manufactures have utilized large inductors (e.g., balun inductors) or isolation transformers to reduce low frequency common mode interference, but these elements are bulky and expensive and are thus typically employed only in highly critical applications.
Another well known approach to achieving acceptable suppression while meeting safety requirements is to incorporate a switching circuit that allows a lower impedance common mode circuit to be used without excessive leakage current. More specifically, it is known to employ a relay-controlled switch to connect an RFI-suppressing capacitor between the neutral conductor and the ground conductor when the AC line voltage (120 Vrms) is detected on the line conductor. If the line voltage is erroneously supplied on the neutral conductor, the relay-controlled switch remains open, and the capacitor is not connected between the neutral and ground connectors, thereby avoiding significant leakage current between the "hot" neutral conductor to the ground conductor. However, if only the capacitor is connected via the switch, the clamping device permanently connected between the neutral conductor and the ground conductor must still be rated for at least 120 Vrms, since its remains connected even under reverse polarity conditions. Thus, including only the capacitor (and not the clamping device) in the switching arrangement results in no improvement in neutral-to-ground common mode transient suppression.
A slight improvement of the conventional switching circuit approach is disclosed in U.S. Pat. No. 5,721,661, incorporated herein by reference in its entirety. In the circuit described therein, the clamping device is connected in parallel with the capacitor between the ground conductor and the switch contact rather than between the ground conductor and the neutral conductor (i.e., the clamping device is switched along with the capacitor instead of being permanently connected). This arrangement allows the clamping circuit to have a lower voltage rating, since it is connected between the ground and neutral conductors only when it has been determined that the AC line voltage is not being carried on the neutral conductor. Further, the capacitor employed in the circuit disclosed in U.S. Pat. No. 5,721,661 has a capacitance of 2.2 micro farads, which purportedly provides RFI suppression superior to that provided by smaller capacitors.
As described in U.S. Pat. No. 5,721,661 and as evidenced by the device disclosed therein, attempts have been made over a long period of time to address the problem of meeting leakage current requirements while attaining better common mode EMI performance at a reasonable cost. However, the power filter circuit disclosed in U.S. Pat. No. 5,721,661 still requires the parallel arrangement of a clamping device and a capacitor between the switch contact of the relay-controlled switch and the ground conductor, and these circuit elements contribute to the cost of the device. Further, while this power filter circuit provides some degree of common mode RFI suppression, the circuit's ability to suppress low frequency interference, such as power line harmonics, is limited, and the circuit is incapable of suppressing common mode transients below 20 volts.
Moreover, certain types of equipment are sensitive to neutral-to-ground voltages; thus, the potential difference across the neutral and ground conductors is often regarded in the industry as a significant measure of circuit performance, with a minimal potential difference being desirable. Unlike more expensive, transformer-based power filter circuits, the presence of the capacitor connected between the neutral and ground conductors in the power filter circuit disclosed in U.S. Pat. No. 5,721,661 results in an undesirable steady-state potential difference between the neutral and ground conductors.
Consequently, there remains a need for an inexpensive power filter circuit which meets UL current leakage requirements under reverse polarity conditions while providing acceptable protection to connected equipment from supply system electromagnetic interference.